1. Field of the Invention
This invention concerns a process for the preparation of carboxylic acids by synthesis gas homologation of aliphatic carboxylic acids by a specific catalyst system.
2. Description of the Prior Art
There is an ever increasing need for a wide variety of aliphatic carboxylic acids of differing carbon numbers and structures which have become important present articles of commerce. The many processes leading to the preparation of these acids include oxidation of saturated and unsaturated hydrocarbons, the carboxylation of mono-olefins, particularly .alpha.-olefins, and dienes such as conjugated dienes like 1,3-butadiene, and the carbonylation of lower aliphatic alcohols.
We now disclose a new preparative route to short-chain aliphatic acids involving the homologation of lower molecular weight aliphatic carboxylic acids. Homologation is effected by treatment of said carboxylic acids with synthesis gas (a mixture of carbon monoxide and hydrogen).
The homologation of carboxylic acids by means of synthesis gas has not, to our knowledge, been disclosed previously. The homologation of saturated alkyl, benzyl alcohols, and substituted benzyl alcohols, by synthesis gas to yield the corresponding higher molecular weight alcohols has been extensively studied. Pertinent examples include the homologation of methanol to ethanol, and the conversion of ethanol to propanol, butanol and pentanol isomers (see: "Carbon Monoxide in Organic Synthesis" by J. Falbe, pages 59-62 and I. Wender, Catal. Rev. Sci. Eng., 14, 97-129 (1976)). Cobalt carbonyls, with or without phosphine or metal modifiers, are commonly used as catalysts in such alcohol homologation reactions (see: L. N. Slaugh, Ger. Offn. No. 2,625,627 and P. D. Taylor, U.S. Pat. No. 4,111,837).
Related homogeneous cobalt carbonyl catalysts are also effective for the synthesis of aliphatic carboxylic acid via the carbonylation of the lower aliphatic alcohols. More recently, soluble rhodium catalysts have become the catalysts of ihorie in, for example, the synthesis of acetic acid via methanol carbonylation (Chem. Tech., p. 605, October 1971).
Other relevant homologation technology includes the recently reported homologation of dimethyl ether and methyl acetate to ethyl acetate (see: G. Braca et. al. 9, Amer. Chem. Soc., 100, 6238 (1978)).
It therefore becomes an object of the invention to provide a novel process of homologation of short-chain aliphatic carboxylic acids to the higher homologues thereof by means of a unique catalyst system. Said process involves the use of synthesis gas as the feedstock along with the acid which is homologized.